1. Field of the Invention
The present invention relates to a method and apparatus for gas phase treating substrates, in particular, a method and apparatus which can treat a large number of substrates in a single operation.
2. Description of the Prior Art
In one prior art gas phase treatment system, substrates to be treated, such as silicon wafers, are supported on a holder of, e.g., quartz and inserted in a reaction tube of quartz. The reaction tube is covered with a cover having a gas inlet port. A reaction gas is fed into the reaction tube from an inlet port and is evacuated through an outlet port, so as to reduce the pressure in the tube to a required level. The substrates are heated by an electrical resistance heater or the like. The reaction gas flowing from the inlet to the outlet is excited by a work coil near the inlet to the plasma state. The excited plasma gas reaches the substrates to be treated, which are heated to a reaction temperature, and treats, for example, deposits on the substrates.
In this system, however the various substrates are located at different distances from where the reaction gas is excited. This makes it difficult to supply active reaction seeds uniformly to all substrates. Therefore, a large number of substrates cannot be treated in a single operation. Further, the lifetime of the excited state of the reaction gas is limited. If the lifetime is too short, active reaction seeds may sometimes reach no substrates.
In an improved prior art system, substrates are held by susceptors made of carbon coated with a silicon nitride (SiC) film and arranged in parallel to each other in a reaction tube, and the tube is surrounded by a work coil at the portion where the susceptors and substrates are placed. Details of this system are described in Nozaki et al., U.S. Pat. No. 4,298,629. In this system, the work coil not only induction heats the susceptors, which heat the substrates by heat conduction, but also excites the reaction gas flowing near the susceptors and substrates. Therefore, the various substrates are all located close to where the reaction gas is excited, so the excited reaction gas can uniformly reach the substrates in the excited state.
A susceptor of, e.g., SiC coated carbon, however, has a low electrical resistance and, therefore, easily carries induction currents, inhibiting excitation of the reaction gas nearby. This makes it necessary to space susceptors holding substrates at a certain distance, decreasing the number of substrates which can be treated in the system in a single operation. Further, disturbances of the electromagnetic field by the work coil, the disturbances being caused by the susceptors, make it difficult to uniformly treat the entire surface of substrates held on the susceptors. Still further, susceptors often tend to be deteriorated by a reaction gas in a reaction tube.